Fertilizer can explode*. We all know that. It was a key ingredient in the bomb that destroyed Oklahoma City's Alfred P. Murrah Federal Building in 1995. Last night, a factory full of the stuff went up with enough force that United States Geological Survey seismographs registered it as a magnitude 2.1 earthquake.

Ammonium nitrate is the chemical that makes these dramatic displays possible. But creating an explosion isn't as simple as just having a pile of ammonium nitrate — let alone a pile of fertilizer — sitting around. We've come to think of this as pretty volatile stuff. But, according to chemist Jimmie Oxley, ammonium nitrate is a lot less dangerous than you might guess. Despite a history of high-profile explosions, like the one that happened last night, ammonium nitrate isn't considered to be that big of a danger. In fact, Oxley called it a "marginal explosive" — a chemical that is mostly safe, but can become dangerous when the conditions are just right.

Oxley studies energetic materials at the University of Rhode Island. You could say that she studies stuff that explodes, but it actually goes a lot further than that. Energetic materials aren't just explosives. The classification includes anything that produces heat as it decomposes. That includes ammonium nitrate, but it also includes your compost pile.

"If you keep a compost heap you might have seen it steaming in the winter sometimes," Oxley said. "And it can even catch on fire. That's because biorganisms break down the compost and release heat. Sometimes, that process releases enough heat that it causes the whole pile to catch."

When it comes to energetic materials, the thing you really want to avoid is a runaway reaction, when a substance starts producing enough heat, on its own, to catch itself on fire and then keep that fire going.

But, amazingly, even that isn't enough to ensure that ammonium nitrate will explode, Oxley said. A couple of years ago, she put together a list of ammonium nitrate accidents that had happened around the world — usually in factories, or during shipping. There are 24 cases on the list that involved fire. Of those, in only 11 cases did the event go from fire to detonation.

In fact, since the 1950s, ammonium nitrate-based explosives have basically supplanted the older dynamite explosives used in mining and other industries, precisely because they are so much safer and harder to detonate. Ammonium nitrate isn't even classed as an explosive, Oxley said.

"It's very difficult to get it to detonate at a reasonable scale," she said. "You can toss 50 pounds of it in the back of your car and it will do nothing. With something like dynamite even a gram or two is highly explosive."

But, obviously, ammonium nitrate does explode sometimes. So what makes those circumstances different?

The most important factor is how much ammonium nitrate you have. Fifty pounds ain't nothing. But a couple hundred tons of the stuff is a different story. If that huge amount of ammonium nitrate also catches on fire ... then you have a problem.

As it burns, ammonium nitrate goes through chemical changes that lead to the production of oxygen. And what does a fire need to keep going and get bigger? Why, oxygen.

The largest industrial accident in the United States happened in 1947, off the coast of Texas City, Texas, when two shipping vessels full of ammonium nitrate exploded. Six hundred people were killed. The explosion might not have happened had the captain of one of the two ships adopted a different tactic for fire-fighting. When he realized his hold was in flames, Oxley said, he had a choice — drown the fire or smother it. The captain opted for smothering it, sealing the hold and trying to starve the fire of oxygen. But the pyre of burning ammonium nitrate was producing its own oxygen. Instead of putting out the flames, the act of sealing the hold allowed the fire to burn bigger and longer without inturruption.

Contrary to some explanations, you don't need a physical jolt to make a great big pile of burning ammonium nitrate explode. The fire alone will do just fine, Oxley said. That's because, depending on how the ammonium nitrate is packed together, the heat can create a sealed plug, trapping hot gases.

Think of a cigarette, she said. When you light it, most of the gas flows away from the cigarette. But some doesn't. That stuff that hangs around helps to pre-warm the parts of the cigarette that haven't already caught fire. That same basic process can happen with a pile of burning ammonium nitrate, only, in that situation, the pre-warmed chemical can end up fusing together. The space behind the plug keeps on being heated and gases form. Hot gas expands, but, behind the plug, it has nowhere to go. Eventually, the gas will break through the seal and the force of that will trigger an explosion.

*Fertilizer can also detonate. Although we laypeople use them as synonyms, "explode" and "detonate" actually mean different things. The force of detonations travels faster than the speed of sound. Explosions don't. Both can still kill people, but a detonation is a lot more likely to cause severe damage to large buildings. Experts will probably be debating for a while whether the West, Texas incident was an explosion or a detonation, Oxley said. Meanwhile, she suspects that the Boston Marathon bombings were likely to have been an explosion.

About the Author

Maggie Koerth-Baker is the science editor at BoingBoing.net. From August 2014-May 2015, she will be a Nieman-Berkman Fellow at Harvard University. You can follow Maggie's adventures in the Ivory Tower by subscribing to The Fellowship of Three Things newsletter.

That seems to be what they’re reporting. Which would be consistent with ‘storage tanks’ (ammonium nitrate is not a liquid), the reported smells, and the company’s filings to the effect that they had no explosive materials. But it’s even harder to get a gigantic explosion from anhydrous ammonia than from ammonium nitrate – it seems like it would require a perfect two-stage explosion, one to mix superheated ammonia with air as a result of tanks bursting, and the second a fuel-air detonation. I haven’t actually heard of such a thing happening. Until now, maybe.
My guess is that they actually did have ammonium nitrate on site (in addition to the tanks of anhydrous ammonia which would have been used to produce it), counter to their regulatory filings, and that it went off. Storing chemicals they weren’t approved for seems more likely than a previously unheard of anhydrous ammonia detonation.

Interesting to note that the plant was built in 1962 and that two of the site’s anhydrous ammonia tanks operated from 1962 to 2006 without a permit. Permits weren’t required at the time, and they had grandfather status until an odour complaint investigation.

If the tanks operated from 1962 to 2006 without requiring a permit, does that mean they weren’t subject to inspection by some governmental agency in that time? When they filed a permit application in 2006, did the tanks need to be inspected prior to issuing the permit?

BLEVE is the initial explosion from the rupture and rapid expansion. That could certainly have happened with an overheated ammonia tank, but the scale of the destruction is too large to be accounted for by the bursting of a tank holding a few tens of tons of ammonia, however overheated they might have been. Maybe you could level the factory building that way, but not the whole town. Now, since ammonia is a fuel I suppose that *in principle* you could mix it with just the right amount of air and then set it off, but from what I’ve read its difficult to get that to work (detonating it all at once, rather than just making a fireball) even with gases that are a whole lot more explosive than ammonia. I think it’s extraordinarily unlikely that such a thing would have happened so perfectly by accident (if it’s even possible); way more likely that they had stashed a batch 100+ tons of ammonium nitrate and that that is what went off.

Was there not a aluminum plant that blew up some years ago (thanks to a fire and sitting on top of a gas line) that had a large stack of barrels of aluminum powder on the grounds because they could not find a buyer for it or something? Could be something similar going on here.

Interesting article, thanks for the science. Your end note about explosions vs. detonations was confusing for me, because I thought detonations were explosions. After some searching, I found a useful article on exponent.com that seems to indicate that a detonation is a type of explosion, the other being a deflagration – http://www.exponent.com/explosions/

The area that exploded was reportedly the “Dry Barn” which housed AN. AN can, in certain circumstances be quite explosive. Usually not by itself, but under high temps (as in the burning enclosure the AN was in, see here: http://www.youtube.com/watch?v=ROrpKx3aIjA) it can most certainly blow up.

Every media outlet is making a big fuss about how this was felt we a MAGNITUDE 2.1 EARTHQUAKE, without mentioning that’s so faint as to be almost imperceptible. I felt a magnitude 4 quake once, and I thought a big truck had gone by.

A 2.1 earthquake really is nothing, but, it does speak to the amount of energy released in the explosion. We already know that the shockwave sent through the atmosphere was quite impressive, since it blew apart buildings and so on. But since most of the energy did go into the atmosphere and not into the ground, registering as a 2.1 earthquake is still pretty impressive.

I couldn’t really speak as to how it compares to other surface explosions; underground nuclear tests and so on certainly would register as bigger though.

I think what it comes down to, though, is that people in most of the country haven’t really felt an earthquake and don’t inherently understand what different magnitudes feel like. So any sort of earthquake is impressive in some sense.

Also, while many people here in California do confuse small earthquakes with passing trucks and so on, if you read up on the science behind earthquake wave propagation and so on it’s pretty easy to tell the difference – earthquake waves create distinctive patterns of motion that can’t really be replicated. You do have to be attuned to this information at all times, though, and I guess non-geologists generally wouldn’t be.

I was near a magnitude 5.1 earthquake in January, which was at 7km depth. My apartment is on the 30th floor, so first I heard a loud sound for a few seconds, like someone was drilling concrete in the next apartment, then for a while it reminded me of being at sea. I was daydreaming at the time, so it didn’t seem that weird until I remembered that I was on land. There were no casualties from the quake or collapsed buildings, but it did freak a lot of people out (and also made cracks in some houses).

That’s what I recall. Its a good oxidizer and a mediocre explosive. Add an additional fuel source and you’ve got a good explosive. Same deal with hydrogen peroxide, it can explode on its own, but usually wont. Give it almost anything that’s nicely flammable and you change the whole deal.

Yes, despite what Maggie says above, ammonium nitrate does not burn by itself, though it decomposes (sometimes explosively) when heated. This produces nitrous oxide, which can itself decompose to produce oxygen which will support combustion.

I can clarify this, but I don’t think I said that it catches fire by itself. It produces heat has it decomposes … this is according to the chemist I spoke with. And when it is burning, it oxidizes (i.e., the bits about producing its own oxygen and feeding a fire that’s already there). It can also explode with the help of just fire.

Can you point me to the spot in the article where it sounds like I’m saying it will just catch fire by itself? I need to adjust that, apparently.

No, they’re usually white to off-white, tending toward yellowish. For the curing salt, they add coloring so that you won’t confuse it with table salt. I suspect the fertilizer is similarly tinted for identification.

despite what the media presstitutes and the govt would have you believe ANFO (ammonium nitrate/fueloil) is a low energy type of explosive, unlike c-4, more of a heaving charge. farmers use this to blow stumps out, some times its used at strip mines to blow whole walls out. very cheap and difficult to get to go off unless you have a good detonator.

Low speed is not quite the same as low energy and sort of orthagonal to low danger and low destruction. Definitely high speed explosions are the ones that asplode everything into itty bitty bits while low speed explosions are the ones that fling huge chunks around… but flinging huge chunks of things around turns everything into plain old kinetic kill impactors. Getting 50 lb chunks of concrete hurled at “low speed explosion” speeds is plenty dangerous.

I don’t see the media or the government calling ANFO a high speed explosive; it is, however, excellent at destroying buildings… which tend to have people in or near them. It’s dangerous. The kind of danger is different from C-4, but that doesn’t make an ANFO bomb SAFE.